WO2022081054A1

WO2022081054A1 - Method and transceiver node for synchronizing in a wireless communication network

Info

Publication number: WO2022081054A1
Application number: PCT/SE2020/050987
Authority: WO
Inventors: Alexandra MIKAELSSON; Obada ALHAJ MOUSSA
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2020-10-16
Filing date: 2020-10-16
Publication date: 2022-04-21

Abstract

Disclosed is a method and a first transceiver node (121) of a wireless communication network (100) for synchronizing in the wireless communication network. The method comprises receiving wirelessly, a first stream comprising first synchronization reference signals from a second transceiver node (122), the first stream comprising a first clock stratum value, and receiving wirelessly a second stream comprising second synchronization reference signals from a third transceiver node (123), the second stream comprising a second clock stratum value. The method further comprising controlling, based on an evaluation of the first and the second clock stratum value, an internal clock of the first transceiver node (121) based on a time-reference value derived from a reception time of at least one of the second synchronization reference signals, only when the second clock stratum value is the same or lower than the first clock stratum value, and controlling based on the evaluation, the internal clock based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals, only when the first clock stratum value is the same or lower than the second clock stratum value.

Description

METHOD AND TRANSCEIVER NODE FOR SYNCHRONIZING IN A WIRELESS COMMUNICATION NETWORK

Technical Field

[0001] The present disclosure relates generally to methods and transceiver nodes for synchronizing in a wireless communication network. More specifically, the present disclosure relates to methods and transceiver nodes of a wireless communication network for transceiver node synchronization in the wireless communication network. The present disclosure further relates to computer programs and carriers corresponding to the above methods and nodes.

Background

[0002] Contemporary wireless communication networks such as Long-Term Evolution (LTE), and future wireless networks based on 5G, such as New Radio (NR) are dependent on network synchronization, i.e. time alignment between transceiver nodes in the network. In practice, the key time alignment requirement applies at an antenna reference point (ARP) between radio access network nodes aka base stations that serve contiguous or overlapping coverage areas. However, there are Timing Errors (TE) appearing for clocks of the transceiver nodes in the network towards a common reference time (CRT) base, i.e. a primary reference clock. The difference in time between transceiver nodes in the network is called a relative Time Error (rTE). There is a synchronization requirement specified between transceiver nodes called relative Time error specification (rTE_s). An example of rTE_s is specified in 3GPP TS36.133 section 7.4 release 15.2.0 as rTE<3ps when operating TDD LTE services with cell radius <= 3km.

[0003] Network Synchronization implementations traditionally seek to minimize the rTE for all transceiver nodes in a network to the CRT, by continuously adjusting each transceiver node’s clock to a local reference that has traceability to the CRT. The CRT may be e.g. GPS System Time. At each transceiver node, the local reference can be provided by a global navigation satellite system (GNSS) receiver that receives a CRT. Alternatively, a CRT is carried over the backhaul network, e.g. via a timing protocol such as Precision Time Protocol (PTP), to the transceiver nodes. Hence the direct network synchronization requirement between transceiver nodes (rTE_s) is achieved by regulating each transceiver node clock to be within error TE_s of the CRT. To define requirements of maximum acceptable time error, the derived relationship between TE_s and rTE_s is as follows: |TE_s| < |rTE_s|/2. This means that to achieve an rTE below for example 3 microseconds, the TE for each transceiver node clock has to be less than 1 ,5 microseconds off the CRT.

[0004] However, there is a growing interest to employ over-the-air (OTA) Time synchronization to synchronize transceiver nodes instead of distributing time synchronization through the backhaul network. In OTA Time Synchronization, the transceiver nodes distribute timing reference signals aka synchronization reference signals over-the air among them. However, if transceiver nodes listen to each other’s transmissions of synchronization reference signals and use the measurements for own synchronization, there inevitably occur timing loops. Timing loops means that a first transceiver node sending out a synchronization reference signal will eventually receive a synchronization reference signal back from another transceiver node that in its turn received a synchronization reference signal that referred to the originally sent synchronization reference signal of the first transceiver node. Such a synchronization reference signal received in a timing loop may risk the synchronization quality. Consequently, there is a need for a solution for avoiding such timing loops.

Summary

[0005] It is an object of the invention to address at least some of the problems and issues outlined above. It is possible to achieve these objects and others by using methods and transceiver nodes as defined in the attached independent claims.

[0006] According to one aspect, a method is provided that is performed by a first transceiver node of a wireless communication network and arranged for synchronizing in the wireless communication network. The method comprises receiving wirelessly, a first stream comprising first synchronization reference signals from a second transceiver node, the first stream comprising an ID of the second transceiver node and a first clock stratum value, and receiving wirelessly, a second stream comprising second synchronization reference signals from a third transceiver node, the second stream comprising an ID of the third transceiver node and a second clock stratum value. The method further comprises controlling, based on an evaluation of the first and the second clock stratum value, an internal clock of the first transceiver node based on a time-reference value derived from a reception time of at least one of the second synchronization reference signals of the second stream, only when the second clock stratum value is the same or lower than the first clock stratum value, and controlling, based on the evaluation, the internal clock of the first transceiver node based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals of the first stream, only when the first clock stratum value is the same or lower than the second clock stratum value.

[0007] According to another aspect, a first transceiver node is provided that is operable in a wireless communication system and configured for synchronizing in the wireless communication network. The first transceiver node comprises a processing circuitry and a memory. Said memory contains instructions executable by said processing circuitry, whereby the first transceiver node is operative for receiving wirelessly, a first stream comprising first synchronization reference signals from a second transceiver node, the first stream comprising an ID of the second transceiver node and a first clock stratum value, and receiving wirelessly, a second stream comprising second synchronization reference signals from a third transceiver node, the second stream comprising an ID of the third transceiver node and a second clock stratum value. The first transceiver node is further operative for controlling, based on an evaluation of the first and the second clock stratum value, an internal clock of the first transceiver node based on a timereference value derived from a reception time of at least one of the second synchronization reference signals of the second stream, only when the second clock stratum value is the same or lower than the first clock stratum value and controlling, based on the evaluation, the internal clock of the first transceiver node based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals of the first stream, only when the first clock stratum value is the same or lower than the second clock stratum value.

[0008] According to other aspects, computer programs and carriers are also provided, the details of which will be described in the claims and the detailed description.

[0009] Further possible features and benefits of this solution will become apparent from the detailed description below.

Brief Description of Drawings

[00010] The solution will now be described in more detail by means of exemplary embodiments and with reference to the accompanying drawings, in which:

[00011] Fig. 1 is a schematic block diagram of a wireless communication network in which the present invention may be used.

[00012] Fig 2 is a flow chart illustrating a method performed by a transceiver node, according to some possible embodiments.

[00013] Fig. 3 is a schematic block diagram of a wireless communication network performing a method according to possible embodiments.

[00014] Fig. 4 is a schematic block diagram of two transceiver nodes performing a synchronization process, according to further possible embodiments.

[00015] Fig. 5 is a block diagram illustrating a transceiver node in more detail, according to further possible embodiments.

Detailed Description

[00016] Fig. 1 describes an example of a wireless communication network 100 that uses over-the-air (OTA) time synchronization. The wireless communication network 100 may be any kind of wireless communication network that can provide radio access to wireless devices. Example of such wireless communication networks are Global System for Mobile communication (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (CDMA 2000), Long Term Evolution (LTE), LTE Advanced, Wireless Local Area Networks (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), WiMAX Advanced, as well as fifth generation wireless communication networks based on technology such as New Radio (NR).

[00017] The wireless communication network 100 comprises a plurality of transceiver nodes 121-129. The plurality of transceiver nodes 121-129 may each be either a radio access network node aka base station or an antenna reference point (ARP). The ARP is the physical point in an antenna at which radiation spreads, e.g. spherically. Typical base station antennas with simple patterns and linear polarizations have a well-defined predictable ARP. In practice, the ARP can be approximated by the physical RF connector port without impact to dependent applications.

[00018] Further, in fig. 1 there is a GNSS receiver 130 that receives information of a Common Reference Time (CRT) from a satellite 140. One transceiver node 122 of the plurality of transceiver nodes 121 -129 is directly connected to the GNSS receiver 130 and thereby functions as a time reference node in the wireless communication network 100. Other known ways of distributing or determining a CRT at a time reference node except for using GNSS receivers may also be applicable. The time reference node 122 wirelessly transmits information of the CRT received from the GNSS receiver as synchronization reference signals in the wireless communication network. A reference signal, such as the synchronization reference signal, is a signal that only exists at the physical layer (PHY). The reference signal is not for delivering any specific information, it is for finding a specific reference point in time. In contrast, data packets, Precision Time Protocol (PTP) packets and Network Time Protocol (NTP) packets contain information that the receiver interprets and reads out from the received data. [00019] The transceiver nodes that are close enough to wirelessly receive the synchronization reference signals, in the example of fig. 1 transceiver nodes 121 , 123 and 124, receives wirelessly the synchronization reference signals from the time reference node 122 and retransmits wirelessly the synchronization reference signals in their turn so that the synchronization reference signals are received at transceiver nodes 125, 126, 127 and 128. Further, to be able to reach transceiver node 129, the synchronization reference signals need to be resent wirelessly one more time from one or more of transceiver nodes 125, 127 and 128. As the transceiver nodes 121 , 123-129 are instructed to resend received synchronization reference signals, there is a risk that there will be a synchronization reference loop in the network. One such loop is from transceiver node 121 to transceiver node 125 to transceiver node 123 and back to transceiver node 121 . If the transceiver node 121 uses a synchronization reference signal that has travelled wirelessly along such a loop instead or together with a synchronization reference signal received directly from the time reference node 122 for controlling its internal clock, the synchronization will fail, or at least be worsened.

[00020] To cater for this synchronization loop problem, the present invention is built on the idea of encoding information of clock stratum values or clock strata in streams comprising the synchronization reference signals. When wirelessly receiving such a stream of synchronization reference signals, a transceiver node can compare the clock stratum value of the received stream of synchronization signals with a clock stratum value of a last used or received stream of synchronization signals, and only use the newly received synchronization signal if the clock stratum value is lower or the same as the last used or received synchronization signal. Hereby, the transceiver node can easily detect how many hops a synchronization signal stream has passed over and discard any possible wireless synchronization reference loops. Observe that before wirelessly resending the received synchronization signal stream, the transceiver node adds the clock stratum value of the received stream with 1 and encodes it into the synchronization reference signal stream it sends further. [00021] A clock stratum value is a value defining a number of hops from a reference clock or synchronization reference source. A number of hops defines how many times a signal was resent after leaving the signal source before it is received by the transceiver node, in other words over how many transceiver nodes the current transceiver node is wirelessly connected to the synchronization reference source. A synchronization reference source and a node directly connected to the synchronization reference source, in the example above timereference node 122, has clock stratum value 0. A node that receives a reference signal over a wireline or wireless connection from a node directly connected to the synchronization reference source has clock stratum value 1 , in the example above transceiver nodes 121 , 123, 124. Clock stratum value (or clock stratum) is a well- known term, see for example https://en.wikipedia.org/wiki/Network Time Protocol#Clock strata that describes clock stratum for use in Network Time Protocol (NTP).

[00022] Fig. 2, in conjunction with fig. 1 , describes a method performed by a first transceiver node 121 of a wireless communication network 100 for synchronizing in the wireless communication network. The method comprises wirelessly receiving 202 a first stream comprising first synchronization reference signals from a second transceiver node, the first stream comprising an ID of the second transceiver node and a first clock stratum value and wirelessly receiving 206 a second stream comprising second synchronization reference signals from a third transceiver node, the second stream comprising an ID of the third transceiver node and a second clock stratum value. The method further comprises controlling 208, based on an evaluation 207 of the first and the second clock stratum value, an internal clock of the first transceiver node 121 based on a time-reference value derived from a reception time of at least one of the second synchronization reference signals of the second stream only when the second clock stratum value is the same or lower than the first clock stratum value, and controlling, based on the evaluation 207, the internal clock of the first transceiver node 121 based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals of the first stream, only when the first clock stratum value is the same or lower than the second clock stratum value. [00023] In other words, the first transceiver node evaluates 207 the wirelessly received first and the second clock stratum value. When the evaluation 207 shows that the second stratum value is the same or lower than the first stratum value, the second synchronization reference signals are used for controlling the internal clock. However, when the evaluation shows that the first stratum value is lower than or the same as the second stratum value, the first transceiver node controls 209 the internal clock based on the first synchronization reference signals. The synchronization reference signals with the highest stratum value are discarded. Further, when the first and the second synchronization reference signals have the same stratum value, both signals may be used, as will be shown below, or alternatively only one of the first or second synchronization reference signals are used.

[00024] By the above method, processing resources of the first transceiver node are spared in case the second stratum value is higher than the first stratum value, as the transceiver node does not have to analyze the second synchronization reference signals, or vice versa. Further, as the synchronization reference signal streams comprise the clock stratum value, no additional message exchange is needed for communicating the clock stratum value, which saves communication interface resource and internal processing resources. Also, timing reference loops between transceiver nodes are prevented. In other words, as the first transceiver node will only start using a synchronization reference signal that has the same or lower clock stratum value than the synchronization reference signal it already uses, the first transceiver node can be sure that the synchronization reference signal used for controlling its internal clock does not originate from itself.

[00025] According to an embodiment, when the second stratum value is the same as the first stratum value, both the second and the first synchronization reference signals may be used for controlling the internal clock. Alternatively, only the second synchronization reference signals or only the first synchronization reference signals may be used. [00026] That the first stream comprises an ID of the second transceiver node and a first clock stratum value may signify that the first clock stratum value and possibly also the second transceiver ID are sent in the first synchronization reference signals. However, as an alternative, the second transceiver ID and possibly also the first clock stratum value may be sent in the stream in between the synchronization reference signals, such as in information of when and over which sub-band the synchronization reference signal is to be sent. The same is valid for the second stream. That the first stream comprises the first clock stratum value and the ID may signify that the first clock stratum value and the ID is encoded into the first stream of first synchronization reference signals. That the second stream comprises the second clock stratum value and the ID may signify that the second clock stratum value and the ID is encoded into the second stream of second synchronization reference signals. It has been agreed between the transceiver nodes of the wireless communication network how to decode the encoded ID and clock stratum value.

[00027] According to an embodiment, the first clock stratum value is received 202 encoded into the first synchronization reference signals, and the second clock stratum value is received 206 encoded into the second synchronization reference signals.

[00028] According to an embodiment, the first clock stratum value is encoded into a root sequence number or sequence initialization value of the first synchronization reference signals, and the second clock stratum value is encoded into a root sequence number or sequence initialization value of the second synchronization reference signals.

[00029] By the above two embodiments, the first transceiver node when receiving a stream of synchronization reference signals can easily detect the clock stratum value of the received stream, compare the clock stratum value to other received clock stratum values and/or its own already existing clock stratum value and decide whether to use this stream for controlling its internal clock or not. The comparing may be performed according to the following: The transceiver node compares the received synchronization reference signal with one or more selfgenerated expected signals that has the code for the clock stratum values that the transceiver node can accept. The comparison functions as a mask so that only if there is a match between the received signal and one of the expected signal(s), the internal clock will be controlled based on a time-reference value derived from a reception time of the received synchronization reference signal. So if the transceiver node is only interested in clock stratum value 0, because it already has synchronized from a signal of stratum value 0 or it has just received a synchronization reference signal with stratum value 0, the transceiver node will generate an expected signal with root sequence number or sequence initialization number corresponding to clock stratum value 0, and if the received signal correlates with the generated expected signal, the synchronization reference signal will be used for controlling the internal clock, otherwise not. If the transceiver node can accept more than one stratum value, the transceiver node will generate more than one expected signal that the received signal is to be compared with, e.g. when stratum value 0 and 1 can be accepted, the transceiver node generates two expected signals with corresponding root sequence number or sequence initialization number.

[00030] According to another embodiment, the method further comprises wirelessly transmitting 210, after receiving the second stream, a third stream comprising third synchronization reference signals, the third stream comprising an ID of the first transceiver node and a third clock stratum value. The third stratum value is the second clock stratum value added by 1 when the second clock stratum value is the same or lower than the first clock stratum value, and the first clock stratum value added by 1 when the second clock stratum value is higher than the first clock stratum value. Consequently, the first transceiver node, which according to the above uses an internal clock controlled based on the synchronization reference signals that has the lowest clock stratum value, transmits synchronization reference signals having one level higher clock stratum value than the synchronization reference signals used to control its internal clock. This makes it clear to other nodes that this synchronization reference signal is one hop further from the primary reference clock than the first or second synchronization reference signals, whichever was the lowest, and should not be used by any of the other transceiver nodes having access to synchronization reference signals of a lower clock stratum value.

[00031] According to another embodiment, when the second clock stratum value is the same as the first clock stratum value, the internal clock of the first transceiver node is controlled 208 based on the first time-reference value derived from the reception time of the at least one of the first synchronization reference signals of the first stream and also on the second time-reference value derived from the reception time of the at least one of the second synchronization reference signals of the second stream. When the first and second clock stratum values are the same, the first transceiver node in this embodiment takes both first and second synchronization reference signals into consideration and can therefore make better decisions and achieve a better synchronization, as two different signals having the same clock stratum value are taken into consideration.

[00032] According to an alternative of the above embodiment, the internal clock of the first transceiver node is controlled 208 in order to minimize error to the first time-reference value and the second time-reference value. Hereby, the timereference values of the streams having the same clock stratum value are used in an almost optimal way. In the same way, time-reference values of more than two streams having the same clock stratum value may be used for controlling the internal clock of the first transceiver node. Error to the first time-reference values and the second time-reference values may be minimized using for example the least square method.

[00033] According to another embodiment, when the first transceiver node loses synchronization, the third stream comprising third synchronization reference signals is wirelessly transmitted 210 with a highest possible clock stratum value. This would signal “do not use” to other transceiver nodes and no transceiver node risks using a non-synchronized reference signal. Still further, the first transceiver node keeps with the used transmission protocol and transmits a reference signal at the prescribed transmission resource even though it is non-synchronized. [00034] Further, it may be so that the first stream of first synchronization reference signals is received well in advance of the second stream of second synchronization reference signals. In such a case, the first transceiver node may control 204 its internal clock based on the first stream already when the second stream arrives. In this case also, the method above applies, i.e. the controlling steps 208, 209 may apply.

[00035] As illustrative examples, referring to fig. 1 , when the first transceiver node mentioned in the methods above is the transceiver node named 121 in fig. 1 , the following will happen: When the first transceiver node wirelessly receives synchronization signals from the time reference transceiver node 122, those synchronization signals will be received together with a clock stratum level 0, as the transceiver node 122 is directly connected to the GNSS receiver 130. The first transceiver node 121 will then discard any synchronization signals wirelessly received from the transceiver nodes 123 and 125 as their transmitted synchronization signals will be accompanied with higher clock stratum levels than 0. Further, the first transceiver node 121 will in its turn wirelessly transmit synchronization signals accompanied with clock stratum level 1 , which is one higher than the clock stratum level of the synchronization signal received from transceiver node 122.

[00036] As another example, when the first transceiver node mentioned in the methods above is transceiver node 127, and transceiver node 123 has lost its synchronization, transceiver node 127 receives a synchronization signal from transceiver node 125. This synchronization signal has clock stratum level 2, (122 to 121 to 125 to 127 = 3 hops). Transceiver node 127 controls its internal clock based on this synchronization signal and in its turn transmits a synchronization signal together with a clock stratum value 3. Then transceiver node 123 gets synchronized again, directly from the time reference transceiver node 122. Consequently, transceiver node 123 receives synchronization signals with clock stratum value 0 from transceiver node 122 and uses those for controlling its internal clock. Further, the transceiver node 122 transmits synchronization signals together with clock stratum value 1 . Those signals are received by transceiver node 127, which compares the clock stratum value of those signals, i.e. 1 , with the clock stratum value of the signals it used last time received from transceiver node 125, which was 2. As a result of the comparison, transceiver node 127 starts using the new synchronization signals with clock stratum value 1 instead of the old ones, for synchronizing its internal clock. Further, transceiver node 127 transmits new synchronization signals together with clock stratum value 2.

[00037] Fig. 3 shows an example of how different transceiver nodes wirelessly transmit synchronization signals together with different stratum values. In this example of wireless communication network 300 there are two different GNSS receivers 330, 350 that receives CRTs from satellites 340, 345. Transceiver node 321 is directly connected to a first GNSS receiver 330 and transceiver node 324 is directly connected to a second GNSS receiver 350. Transceiver nodes 321 and 324 therefore transmits synchronization signals appended with stratum value 0. Transceiver nodes 322, 323 and 324 receive those synchronization signals and uses them for synchronizing their internal clocks. As a result, they transmit synchronization signal appended with stratum value 1. Transceiver node 325 on the other hand has lost synchronization and therefore transmits synchronization signals appended with the highest possible stratum value, here symbolized with “Do not use”. As soon as transceiver node 325 gets synchronized again, e.g. from received synchronization signals from transceiver node 323, it can control its clock with those synchronization signals and in its turn start transmitting synchronization signals with stratum value 2 (1 +1 ). Further, it may be possible that transceiver node 321 loses its synchronization, this may happen if e.g. GNSS receiver 330 stops receiving synchronization signals from satellites. In this case, transceiver 321 will send synchronizations signals appended with the highest possible clock stratum value “Do not use”. All transceiver nodes 321-326 then have to synchronize to signals originating from transceiver node 324.

[00038] Fig. 4 shows a time-reference node aka synchronization anchor node 401 , such as transceiver node 122 in fig. 1 and another transceiver node 402 such as transceiver node 121 in fig. 1 . The synchronization anchor node 401 wirelessly transmits a synchronization reference signal, e.g. Radio Interface-Based Synchronization (RIBS) reference signal (RS) encoded with stratum value 0 to the transceiver node 402. Before the transceiver node 402 obtains its synchronization, it transmits RIBS-RS with encoded stratum value “Do not use”. However, when the transceiver node 402 wirelessly receives the RIBS-RS with encoded stratum value 0 it measures its Local Time of Arrival (ToA). Further, the anchor node 401 measures ToA on the RIBS-RS encoded with stratum value “Do not use” that the transceiver node 402 sends. Information on this remote ToA is then sent by the synchronization anchor node 401 over backhaul to transceiver node 402. Further, as a schedule for wireless transmission of synchronization reference signals is agreed in advance between the involved transceiver nodes, the transceiver node 402 knows when the synchronization anchor node 401 sent its RIBS-RS with encoded stratum value 0. Based on local ToA, remote ToA and knowledge of sent time points for the RIBS-RSs, transceiver node 402 can calculate signal propagation time over the air. When signal propagation time is known, transceiver node 402 can subtract the signal propagation time from ToA and calculate TE in a TE calculation unit 405. The TE can be used for controlling an internal clock 407 of transceiver node 402 via synchronization block 406.

[00039] Fig. 5, describes a first transceiver node 121 operable in a wireless communication system 100 configured for synchronizing in the wireless communication network. The first transceiver node 121 comprises a processing circuitry 603 and a memory 604. Said memory contains instructions executable by said processing circuitry, whereby the first transceiver node 121 is operative for wirelessly receiving a first stream comprising first synchronization reference signals from a second transceiver node, the first stream comprising an ID of the second transceiver node and a first clock stratum value, and wirelessly receiving a second stream comprising second synchronization reference signals from a third transceiver node, the second stream comprising an ID of the third transceiver node and a second clock stratum value. The first transceiver node 121 is further operative for controlling, based on an evaluation of the first and the second clock stratum value, an internal clock of the first transceiver node 121 based on a timereference value derived from a reception time of at least one of the second synchronization reference signals of the second stream, only when the second clock stratum value is the same or lower than the first clock stratum value and controlling, based on the evaluation, the internal clock of the first transceiver node 121 based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals of the first stream, only when the first clock stratum value is the same or lower than the second clock stratum value.

[00040] According to an embodiment, the first transceiver node 121 is operative for receiving the first clock stratum value encoded into the first synchronization reference signals, and for receiving the second clock stratum value encoded into the second synchronization reference signals.

[00041] According to another embodiment, the first clock stratum value is encoded into a root sequence number or sequence initialization value of the first synchronization reference signals, and the second clock stratum value is encoded into a root sequence number or sequence initialization value of the second synchronization reference signals.

[00042] According to another embodiment, the first transceiver node 121 is further operative for wirelessly transmitting, after receiving the second stream, a third stream comprising third synchronization reference signals, the third stream comprising an ID of the first transceiver node and a third clock stratum value, which is the second clock stratum value added by 1 when the second clock stratum value is the same or lower than the first clock stratum value, and the first clock stratum value added by 1 when the second clock stratum value is higher than the first clock stratum value.

[00043] According to yet another embodiment, when the second clock stratum value is the same as the first clock stratum value, the first transceiver node is operative for controlling its internal clock based on the first time-reference value derived from the reception time of the at least one of the first synchronization reference signals of the first stream and also on the second time-reference value derived from the reception time of the at least one of the second synchronization reference signals of the second stream. [00044] According to an alternative of the above embodiment, the first transceiver node 121 is operative for controlling its internal clock in order to minimize error to the first time-reference value and the second time-reference value.

[00045] According to another embodiment, when the first transceiver node 121 loses synchronization, the first transceiver node is operative for transmitting the third stream comprising third synchronization reference signals with a highest possible clock stratum value.

[00046] According to other embodiments, the first transceiver node 121 may further comprise a communication unit 602, which may be considered to comprise conventional means for wireless communication with other transceiver nodes, such as a transceiver for wireless transmission and reception of signals in the communication network. The communication unit 602 may also comprise conventional means for backhaul communication with other transceiver nodes of the wireless communication network 100. The instructions executable by said processing circuitry 603 may be arranged as a computer program 605 stored e.g. in said memory 604. The processing circuitry 603 and the memory 604 may be arranged in a sub-arrangement 601 . The sub-arrangement 601 may be a microprocessor and adequate software and storage therefore, a Programmable Logic Device, PLD, or other electronic component(s)/processing circuit(s) configured to perform the methods mentioned above. The processing circuitry 603 may comprise one or more programmable processor, application-specific integrated circuits, field programmable gate arrays or combinations of these adapted to execute instructions.

[00047] The computer program 605 may be arranged such that when its instructions are run in the processing circuitry, they cause the first transceiver node 121 to perform the steps described in any of the described embodiments of the first transceiver node 121 and its method. The computer program 605 may be carried by a computer program product connectable to the processing circuitry 603. The computer program product may be the memory 604, or at least arranged in the memory. The memory 604 may be realized as for example a RAM (Random-access memory), ROM (Read-Only Memory) or an EEPROM (Electrical Erasable Programmable ROM). In some embodiments, a carrier may contain the computer program 605. The carrier may be one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or computer readable storage medium. The computer- readable storage medium may be e.g. a CD, DVD or flash memory, from which the program could be downloaded into the memory 604. Alternatively, the computer program may be stored on a server or any other entity to which the first transceiver node 121 has access via the communication unit 602. The computer program 605 may then be downloaded from the server into the memory 604.

[00048] Although the description above contains a plurality of specificities, these should not be construed as limiting the scope of the concept described herein but as merely providing illustrations of some exemplifying embodiments of the described concept. It will be appreciated that the scope of the presently described concept fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the presently described concept is accordingly not to be limited. Reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural and functional equivalents to the elements of the abovedescribed embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed hereby. Moreover, it is not necessary for an apparatus or method to address each and every problem sought to be solved by the presently described concept, for it to be encompassed hereby. In the exemplary figures, a broken line generally signifies that the feature within the broken line is optional.

Claims

1 . A method performed by a first transceiver node (121 ) of a wireless communication network (100) for synchronizing in the wireless communication network, the method comprising: receiving (202) wirelessly, a first stream comprising first synchronization reference signals from a second transceiver node, the first stream comprising an ID of the second transceiver node and a first clock stratum value; receiving (206) wirelessly, a second stream comprising second synchronization reference signals from a third transceiver node, the second stream comprising an ID of the third transceiver node and a second clock stratum value; controlling (208), based on an evaluation of the first and the second clock stratum value, an internal clock of the first transceiver node (121 ) based on a timereference value derived from a reception time of at least one of the second synchronization reference signals of the second stream, only when the second clock stratum value is the same or lower than the first clock stratum value, and controlling (209), based on the evaluation of the first and the second clock stratum value, the internal clock of the first transceiver node (121 ) based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals of the first stream, only when the first clock stratum value is the same or lower than the second clock stratum value.

2. Method according to claim 1 , wherein the first clock stratum value is received (202) encoded into the first synchronization reference signals, and the second clock stratum value is received (206) encoded into the second synchronization reference signals.

3. Method according to claim 2, wherein the first clock stratum value is encoded into a root sequence number or sequence initialization value of the first synchronization reference signals, and the second clock stratum value is encoded into a root sequence number or sequence initialization value of the second synchronization reference signals.

4. Method according to any of the preceding claims, further comprising: transmitting (210) wirelessly, after receiving the second stream, a third stream comprising third synchronization reference signals, the third stream comprising an ID of the first transceiver node and a third clock stratum value, which is the second clock stratum value added by 1 when the second clock stratum value is the same or lower than the first clock stratum value, and the first clock stratum value added by 1 when the second clock stratum value is higher than the first clock stratum value.

5. Method according to any of the preceding claims, wherein when the second clock stratum value is the same as the first clock stratum value, the internal clock of the first transceiver node is controlled (208) based on the first time-reference value derived from the reception time of the at least one of the first synchronization reference signals of the first stream and also on the second timereference value derived from the reception time of the at least one of the second synchronization reference signals of the second stream.

6. Method according to claim 5, wherein the internal clock of the first transceiver node is controlled (208) in order to minimize error to the first timereference value and the second time-reference value.

7. Method according to any of claims 4-6, wherein when the first transceiver node loses synchronization, the third stream comprising third synchronization reference signals is transmitted (210) with a highest possible clock stratum value.

8. A first transceiver node (121 ) operable in a wireless communication system (100) configured for synchronizing in the wireless communication network, the first transceiver node (121 ) comprising a processing circuitry (603) and a memory (604), said memory containing instructions executable by said processing circuitry, whereby the first transceiver node (121 ) is operative for: receiving wirelessly, a first stream comprising first synchronization reference signals from a second transceiver node, the first stream comprising an ID of the second transceiver node and a first clock stratum value; receiving wirelessly, a second stream comprising second synchronization reference signals from a third transceiver node, the second stream comprising an ID of the third transceiver node and a second clock stratum value; controlling, based on an evaluation of the first and the second clock stratum value, an internal clock of the first transceiver node (121 ) based on a timereference value derived from a reception time of at least one of the second synchronization reference signals of the second stream, only when the second clock stratum value is the same or lower than the first clock stratum value, and controlling, based on the evaluation of the first and the second clock stratum value, the internal clock of the first transceiver node (121) based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals of the first stream, only when the first clock stratum value is the same or lower than the second clock stratum value.

9. First transceiver node (121 ) according to claim 8, operative for receiving the first clock stratum value encoded into the first synchronization reference signals, and for receiving the second clock stratum value encoded into the second synchronization reference signals.

10. First transceiver node (121 ) according to claim 9, wherein the first clock stratum value is encoded into a root sequence number or sequence initialization value of the first synchronization reference signals, and the second clock stratum value is encoded into a root sequence number or sequence initialization value of the second synchronization reference signals.

11 . First transceiver node (121 ) according to any of claims 8-10, further being operative for:

Transmitting wirelessly, after receiving the second stream, a third stream comprising third synchronization reference signals, the third stream comprising an ID of the first transceiver node and a third clock stratum value, which is the second clock stratum value added by 1 when the second clock stratum value is the same or lower than the first clock stratum value, and the first 21 clock stratum value added by 1 when the second clock stratum value is higher than the first clock stratum value.

12. First transceiver node (121 ) according to any of claims 8-11 , wherein when the second clock stratum value is the same as the first clock stratum value, the first transceiver node is operative for controlling its internal clock based on the first time-reference value derived from the reception time of the at least one of the first synchronization reference signals of the first stream and also on the second time-reference value derived from the reception time of the at least one of the second synchronization reference signals of the second stream.

13. First transceiver node (121) according to claim 12, operative for controlling its internal clock in order to minimize error to the first time-reference value and the second time-reference value.

14. First transceiver node (121 ) according to any of claims 11-13, wherein when the first transceiver node loses synchronization, the first transceiver node is operative for transmitting the third stream comprising third synchronization reference signals with a highest possible clock stratum value.

15. A computer program (605) comprising instructions, which, when executed by at least one processing circuitry of a first transceiver node (121 ) of a wireless communication network (100), configured for synchronizing in the wireless communication network, causes the first transceiver node (121 ) to perform the following steps: receiving wirelessly, a first stream comprising first synchronization reference signals from a second transceiver node, the first stream comprising an ID of the second transceiver node and a first clock stratum value; receiving wirelessly, a second stream comprising second synchronization reference signals from a third transceiver node, the second stream comprising an ID of the third transceiver node and a second clock stratum value, controlling, based on an evaluation of the first and the second clock stratum value, an internal clock of the first transceiver node (121 ) based on a time- 22 reference value derived from a reception time of at least one of the second synchronization reference signals of the second stream, only when the second clock stratum value is the same or lower than the first clock stratum value, and controlling, based on the evaluation of the first and the second clock stratum value, the internal clock of the first transceiver node (121) based on a time-reference value derived from a reception time of at least one of the first synchronization reference signals of the first stream, only when the first clock stratum value is the same or lower than the second clock stratum value.

16. A carrier containing the computer program (605) according to claim 15, wherein the carrier is one of an electronic signal, an optical signal, a radio signal, an electric signal or a computer readable storage medium.